Aircraft of generally conventional design and shape obtain directional stability primarily from a vertical fin mounted on the aft end of the fuselage, which fin produces an aerodynamic restoring force to return the aircraft to a straight course whenever yawing conditions are encountered during flight.
Since the location of the vertical fin places it in the wakes, i.e., regions of disturbed flow, that form behind the wings and fuselage in flight, the effectiveness of the fin to perform its function is impaired thereby, particularly at the higher angles of attack wherein these regions of disturbance grow relatively large. At and above the wing stall angle of attack the turbulent flow from the wing, coupled with interference from the fuselage, increases to critical proportions that can totally eliminate the effectiveness of the fin resulting in troublesome and even dangerous directional instability of the aircraft. Moreover, at these post stall angles of attack, body effects begin to exert an aerodynamic influence on the total stability of the aircraft. The forebody of the fuselage generates a vortex system, analogous to that produced by a circular cylinder in two dimensional flow, which vortices can have an undesirable effect on the stability of the aircraft. The strength and orientation of the nose vortices are functions of the forebody fineness ratio, i.e., ratio of length to diameter, cross-sectional shape, nose bluntness, and planform shape.
By "nose" we mean the forwardmost tip or leading edge of the fuselage. By "forebody" we mean the section of the fuselage extending from the nose to the region forwardly of the cockpit and leading edges of the wings. The overall effect of the nose vortices can be either stabilizing or destabilizing, depending on the effects of disturbances on the vortex formations.
We have found that by appropriately shaping the nose and forebody, the arrangement or pattern of the nose vortices can be controlled to enhance the directional stability of the aircraft over a much greater range of angle of attack than that produced by the vertical fin per se, and thus significantly increase the operational capability of the aircraft.